1. Field of the Invention
The present invention generally relates to fracturing fluids and, more particularly, but not by way of limitation, to an aqueous based zirconium (IV) crosslinked guar fracturing fluid and a method of making and use therefor, suitable to the purposes of hydraulically fracturing subterranean formations having static bottom-hole temperatures greater than about 250xc2x0 F.
2. Description of the Related Art
An aqueous based crosslinked polygalactomannan fluid is typically used to perform a hydraulic fracturing treatment of a hydrocarbon bearing reservoir when the static bottom-hole temperature of a well exceeds approximately 250xc2x0 F.
One type of aqueous based crosslinked polygalactomannan fluid that might be used in high temperature wells are borate crosslinked guar fluids, where the pH of the crosslinked fluid under treating conditions ranges from about 8.5 to about 12. Typical borate crosslinked guar fluids include, but are not limited to, those described in U.S. Pat. No. 5,145,590 (Dawson), U.S. Pat. No. 5,445,223 (Nelson et al), and in xe2x80x9cChemistry and Rheology of Borate-Crosslinked Fluids at Temperatures to 300xc2x0 F., P.C. Harris, J. Petroleum Technology, March 1993, pp. 264-269.
A second type of aqueous based crosslinked polygalactomannan fluid that might be used in fracturing high temperature wells are titanium (IV) or zirconium (IV) crosslinked derivatized guar fluids where the pH of the crosslinked fluid under treating conditions may range from about 3.5 to about 11. Typical derivatized guar polymers suitable to formulate a titanium (IV) or a zirconium (IV) crosslinked derivatized guar fluid include but are not limited to, alkyl-derivatives, such as hydroxypropyl guar (HPG), carboxyalkyl-derivatives, such as carboxymethyl guar (CMG), and carboxyalkyl-hydroxyalkyl-derivatives, such as carboxymethylhydroxypropyl guar (CMHPG). The guar used in the high temperature borate fracturing fluids is distinguishable from the derivatized guar used in the zirconium (IV) or titanium (IV) high temperature crosslinked fracturing fluid, in that guar for the borate crosslinked fluid is not subjected to a chemical treatment wherein some form of molecular substitution, such as alkylation, carboxylation or combinations thereof, has been performed to derivatize the guar. Typical titanium (IV) or zirconium (IV) crosslinked derivatized guar fluids include but are not limited to, those described in U.S. Pat. No. 3,888,312 (Tiner et al.), U.S. Pat. No. 4,534,870 (Williams), U.S. Pat. No. 4,686,052 (Baranet et al.), and U.S. Pat. No. 4,799,550 (Harris et al.)
In hydraulically fracturing a hydrocarbon bearing reservoir with an aqueous based crosslinked guar or derivatized guar fracturing fluid, it is often necessary that the aqueous based crosslinked guar or derivatized guar fracturing fluid exhibit more than an hour of stability. Those of ordinary skill in the art generally regard stability as a minimum viscosity achieved based upon an agreed rheological test method for an agreed period of time. The agreed rheological test method may be one proposed by the American Petroleum Institute (API) or an adaptation of an API method as proposed by a petroleum production company or its hydraulic fracturing service provider. Alternatively, the agreed rheological test method may be a novel method proposed and mutually agreed upon by the participants in the hydraulic fracturing fluid evaluation and may follow a regime based upon anticipated field conditions.
A borate crosslinked guar fracturing fluid is typically used when bottom-hole temperatures (BHT""s) of a well do not exceed about 325xc2x0 F. and where more than an hour of stability is necessary. A borate crosslinked guar fracturing fluid is generally made more stable by raising the pH of the fracturing fluid, increasing the borate concentration, or increasing the guar concentration in solution. A problem experienced in stabilizing the high temperature borate crosslinked fracturing fluid by raising the pH of the fluid, increasing the borate concentration, or increasing the guar concentration in solution is that pH, borate concentration, or guar concentration can be excessive, any of which may render the fracturing fluid unsuitable for use in the intended hydraulic fracturing treatment of a hydrocarbon bearing reservoir. Increasing chemical constituent loading to accommodate wells with higher BHT""s is also more costly.
A zirconium (IV) crosslinked derivatized guar fracturing fluid is typically used when the BHT of a well does not exceed about 400xc2x0 F. and where more than an hour of stability is necessary. A zirconium (IV) crosslinked derivatized guar fracturing fluid is generally made more stable by raising the pH of the fracturing fluid, increasing the zirconium (IV) concentration, or increasing the derivatized guar concentration in solution. A problem experienced in stabilizing the high temperature zirconium (IV) crosslinked fracturing fluid by raising the pH of the fluid, increasing the zirconium (IV) concentration, or increasing the derivatized guar concentration in solution is that pH, zirconium (IV) concentration, or derivatized guar concentration can be excessive, any of which may render the fracturing fluid unsuitable for use in the intended hydraulic fracturing treatment of a hydrocarbon bearing reservoir. Likewise, as is the case of borate crosslinked fluids, increasing chemical constituent loading to accommodate wells with higher BHT""s is also more costly.
A zirconium (IV) crosslinked derivatized guar fracturing fluid provides certain advantages over a borate crosslinked guar fracturing fluid, especially when BHT of a well exceeds about 325xc2x0 F. At BHT""s less than about 250xc2x0 F., a zirconium (IV) crosslinked derivatized guar fracturing fluid is usable at a pH of less than about 8.5, which is particularly advantageous if carbon dioxide comprises a portion of the fracturing fluid because the pH of the fracturing fluid may be as low as 3.5. Also, at BHT""s greater than about 250xc2x0 F., and certainly as BHT""s increase above about 325xc2x0 F., it is much easier and less costly to delay the gelation of a zirconium (IV) crosslinked derivatized guar fracturing fluid than a borate crosslinked guar fracturing fluid.
Although a zirconium (IV) crosslinked derivatized guar fracturing fluid provides certain operational advantages over a borate crosslinked guar fracturing fluid, a borate crosslinked guar fracturing fluid provides the advantage that it employs guar, which is less costly and more readily available than derivatized guar, and borate crosslinked guar fracturing fluids have historically been shown to operate to temperatures approximately 50xc2x0 F. higher than zirconium (IV) crosslinked guar fracturing fluids. Consequently, a zirconium (IV) crosslinked guar fracturing fluid is typically not suitable for hydraulic fracturing treatments where the BHT of a well exceeds about 275xc2x0 F. In those instances where a zirconium (IV) crosslinked guar fracturing fluid may be utilized where the BHT of a well exceeds 275xc2x0 F., the zirconium (IV) crosslinked guar fracturing fluid requires guar loadings in excess of 50 pounds of guar per 1000 gallons of make-up water and may often require as much as 80 pounds of guar per 1000 gallons of make-up water before a stable fracturing fluid is achieved. When such guar loadings are necessary, derivatized guar is utilized because, although it is more expensive, the derivatized guar requires considerably lower loading levels, and by virtue of the lower polymer loading, the spent fracturing fluid is easier to recover following the fracturing treatment.
Brannon et aL (U.S. Pat. No. 4,801,389) describe partially successful attempts to develop a zirconium (IV) crosslinked guar fracturing fluid for high temperature applications. Brannon et al. teaches to a fracturing fluid pH of about 8 to about 10. Any greater pH in this method tends to accelerate the crosslink rate thereby negating the benefit of delayed crosslinking on the subsequent rheological performance reported for the crosslinked fluid. Thus Brannon et al. were constrained to using sodium bicarbonate only as a buffering agent. This results in the disadvantage that the fluid has a lower pH than a fluid made with the corresponding carbonate. Additionally, the quantity of bicarbonate causes the onset of crosslinking to be substantially delayed whereas the presence of carbonate does not cause a substantial delay. This reduces the flexibility of the fluid since typically it is desirable to be able to vary the crosslink time of fracturing fluids. More flexibility in the control of crosslink time can be available with a blended buffer of bicarbonate and carbonate, where the overall bicarbonate loading can be varied. Furthermore, it is recognized that a greater pH, to pH 12 or higher, may substantially improve the rheological performance of this fluid. Because of the use of bicarbonate alone to control the crosslinking rate does not allow for the fluid pH to rise much above 9, fluids with bicarbonate buffering alone are not suitable for very high temperature wells. Additionally, Brannon et al. is apparently limited to a rheological performance (xe2x89xa7100 cP) of about 0.75 hrs at 325xc2x0 F., whereas most fracturing fluid treatments at this elevated temperature require  greater than 2 hrs performance at xe2x89xa7100 cP. Accordingly, there is a recognized and long felt need for a stable zirconium (IV) crosslinked fracturing fluid formulated utilizing guar at loadings at least comparable to the loadings of derivatized guar in a zirconium (IV) crosslinked derivatized guar fracturing fluid and utilizing a crosslinking delay agent that is not constrained by the pH of the buffer used.
In accordance with the present invention, an aqueous based zirconium (IV) crosslinked guar fracturing fluid having a pH from about 9 to about 12 includes a polymer solution and a zirconium (IV) crosslinking agent in an amount from about 0.1 gallons per thousand gallons(GPTG) to about 5 GPTG of the polymer solution. The polymer solution includes an aqueous fluid, natural guar gum in an amount from about 10 pounds per thousand gallons(PPTG) to about 100 PPTG of the aqueous fluid, a stabilizer in an amount from about 1 PPTG to about 50 PPTG of the aqueous fluid, a alkaline buffer in an amount from about 1 PPTG to about 40 PPTG of the aqueous fluid, and an alpha-hydroxycarboxylic acid delaying agent in an amount from about 0.25 PPTG to about 3.75 PPTG of the aqueous fluid. This crosslinker is typically applied as an aqueous or alcohol solution of the metal.
The aqueous fluid includes substantially any aqueous fluid that does not adversely react with one of the constituents of the fracturing fluid, the subterranean formation, and the fluids present therein. The aqueous fluid is selected from the group consisting of fresh water, natural brines, and artificial brines, such as potassium chloride solutions and sodium chlorides solutions.
The stabilizer includes any free-radical-scavenging compound and is selected from the group consisting of sodium thiosulfate, thiourea, urea, sodium sulfite, and methanol. The pH buffer includes one or more alkaline compounds selected from the group consisting of amines, or ammonium/amine, or alkali metal hydroxides, carbonates, and bicarbonates, such as ammonium hydroxide, potassium carbonate, and sodium bicarbonate, and thioethanolamine.
The delaying agent includes an alpha-hydroxycarboxylic acid selected from the group consisting of citric acid, malic acid, glycolic acid, lactic acid, tartaric acid, gluconic acid, glyceric acid, and mandelic acid. The delaying agent further includes an ammonium, amine, or alkali metal salt of an alpha-hydroxycarboxylic acid, such as sodium citrate and ammonium lactate.
The use of a combination of buffers together with the use of an alpha-hydroxycarboxylic acid or its salt, offers an advantage over Brannon et al., (U.S. Pat. No. 4,801,389) in that the crosslink delay time of the newly proposed fluid, is controllable over a wider pH range.
The zirconium (IV) crosslinking agent includes any zirconium compound or mixture of zirconium (IV) compounds, capable of solubilizing in an aqueous polymer solution to release the metal so that gelation takes place under controlled conditions. The zirconium (IV) crosslinking agent is selected from the group consisting of aqueous or alcohol solutions of complexes of alpha-hydroxycarboxylic acids, monoalkylamine zirconium compounds, dialkylamine zirconium compounds, trialkylamine zirconium compounds, and zirconium alkanolamine complexes. More precisely, the zirconium (IV) crosslinking agent may be selected from the group consisting of zirconium ammonium lactate, zirconium di or tri ethanolamine lactate, zirconium diisopropylamine lactate, zirconium sodium lactate salts, zirconium citrate, zirconium tartate, and zirconium glycolate, and like complexes, and mixtures thereof.
In a method for formulating an aqueous based zirconium (IV) crosslinked guar fracturing fluid having a pH from about 9 to about 12, an aqueous fluid is placed into a suitable mixing device. A stabilizer is added to the aqueous fluid in an amount from about 1 PPTG to about 50 PPTG of the aqueous fluid and allowed to dissolve into the aqueous fluid for a first predetermined period. Natural guar gum is added to the mixture in an amount from about 10 PPTG to about 100 PPTG of the aqueous fluid, and the mixture is mixed for a second predetermined period. An alkaline buffer is added to the mixture in an amount from about 1 PPTG to about 40 PPTG of the aqueous fluid, and the mixture is mixed for a third predetermined period. An alpha-hydroxycarboxylic acid delaying agent is added to the mixture in an amount from about 0.25 PPTG to about 3.75 PPTG of the aqueous fluid, and the mixture is mixed for a fourth predetermined period, thereby forming a polymer solution. A zirconium (IV) crosslinking agent is added in an amount from about 0.1 GPTG to about 5 GPTG of the formulated polymer solution, and mixing is performed until gelation occurs, thereby producing the fracturing fluid.
In a method of hydraulically fracturing a subterranean zone penetrated by a wellbore, an aqueous based zirconium (IV) crosslinked guar fracturing fluid having a pH from about 9 to about 12 is prepared. The fracturing fluid is pumped into the subterranean zone via the wellbore and permitted to gel after having substantially traversed the well bore or after having entered the subterranean formation, thereby causing hydraulic fracturing of the subterranean formation. Proppants may be added to the fracturing fluid, which is utilized to disperse the proppants throughout the subterranean formation. A breaker may be added to the fracturing fluid to permit the removal of the fracturing fluid from the subterranean formation.
It is therefore an object of the present invention to provide an aqueous based zirconium (IV) crosslinked guar fracturing fluid formulated utilizing guar at loadings at least comparable to the loadings of derivatized guar in a zirconium (IV) crosslinked derivatized guar fracturing fluid.
Still other objects, features, and advantages of the present invention will become evident to those of ordinary skill in the art in light of the following.